In British English you'd be wrong for using an em-dash in those places, with most grammar recommendations being for an en-dash, often with spaces.
It's be just as wrong as using an apostrophe instead of a comma.
Grammar is often wooly in a widely used language with no single centralised authority. Many of the "Hard Rules" some people thing are fundamental truths are often more local style guides, and often a lot more recent than some people seem to believe.
